Multiple antennas technology (MIMO, Multiple Input Multiple Output) is one of the most important technologies of modern communication systems. Among the multiple antennas technologies, spatial diversity attracts more attention, since it exploits the independent fading between the antennas to overcome the severe attenuation of signal between a transmit end and a receiving end, and provides a more reliable signal, especially for control signaling; which requires more reliability than usual data signals. This spatial diversity gain is obtained via a space-time block codes (STBC) scheme.
The first STBC is an Alamouti scheme, which is a full-diversity (achieves the maximum diversity order, i.e., diversity order 2 with 2 transmit antennas) and full-rate (normalized rate of 1 symbol/s/Hz) over 2 transmit antennas and 2 symbols time slot. An Alamouti STBC scheme is generalized by using orthogonal design theory for more than 2 transmit antennas. These STBC schemes have full-diversity gain and a simple linear decoding algorithm. However, for complex modulation constellations, such as QAM and PSK, the orthogonal design theory based STBC have the maximum rate 0.75 (¾), thus the transmission rate is less. Moreover, it has been proven theoretically that for complex constellations, an Almouti scheme is the unique scheme with full-rate, full-diversity and a simple linear decoding algorithm at the same time.
Another approach of STBC is pre-coded STBC. These schemes try to achieve full-rate and full-diversity gain for more than 2 transmit antennas, but at the cost of simple linear decoding complexity being lost. To achieve full-diversity, a full maximum likelihood (ML) decoding algorithm should be used. The complexity of an ML algorithm is exponential with the number of transmit antennas and constellation size. This high complexity makes it is impractical to use this STBC scheme, especially for high modulation types and more than 2 transmit antennas.
Thus, a strong need exists for a full-rate, full-diversity space-time block code technique for multiple transmissions using simple linear decoding complexity.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.